This application joins two physician-scientist leaders in the use of genetically engineered mouse models (GEMMs) to evaluate the utility of these animals for preclinical studies that can directly impact human clinical trials. We will investigate the biology of therapeutic response in solid tumors with particular interest in the PI3-kinase (PI3K) pathway and cells that are resistant to standard of care treatments. Based on our expertise, we will be comparing and contrasting three tumor types where the PI3K pathway contributes to oncogenesis and therapeutic resistance, namely medulloblastoma, glioma, and prostate cancer. We anticipate similarities and differences between these tumor types and expect all three to be cross-informative. In the first project we will compare several drug combinations to determine optimal strategies for complete blockade of this signaling pathway in these tumors in vivo. The second project will investigate the character of the cells that survive radiation in the case of brain tumors and androgen depletion (castration) in the case of prostate cancer, and determine what role the PI3K pathway plays in their stem-like and resistant properties. In Project 3, we will use these mouse models to determine the impact of additional genomic alterations on treatment response and identify mRNA-based gene expression signatures as predictive biomarkers that could then be taken from the mouse into human trials. Collectively, the three Projects bring together experts in mouse modeling, experimental therapeutics, comparative pathology and translational medicine and provide a fertile environment for developing the careers of several promising young physician-scientists.